1. Field of the Invention
This invention is directed to polymers used in medical applications, such as coatings for medical devices or implantable prostheses.
2. Description of the State of the Art
Among the many advances in medical practice in recent years is the development of medical devices that supplement the surgeon's skills. Examples of these are a variety of vascular catheters and guide wires that can be used to treat remote areas of the circulatory system otherwise available only by major surgery. Another is a stent, a device that retards restenosis after angioplasty. Another is the intra-ocular lens that restores youthful eyesight to the elderly afflicted with cataracts. Heart valves, artificial pacemakers, and orthopedic implants are among a lengthening list.
Nearly all of the above-described devices are constructed of plastics and metals that were never intended to invade, and sometimes reside for prolonged periods, in the human body. These devices present surfaces that bear little or no semblance to those of the human organs, which are generally hydrophilic, slippery, and obviously biocompatible. The penalty imposed on invasive devices that are not biocompatible is that they tend to be treated as foreign objects by the body's immune system. Inflammation and thrombosis often result.
The surface of devices already designed and manufactured from such materials can be made biocompatible, as well as hydrophilic and slippery, by properly designed coatings. Thus, the way has been opened to construct medical devices from conventional plastics and metals having the particular physical properties required, and then to apply suitable coatings to impart the desired properties to their surfaces.
In addition to improving the biocompatibility of the surface, polymers can be used to deliver biologically or pharmaceutically active agents to a treatment site. For example, stents have been modified with polymers for local application of a therapeutic substance. In order to provide an effective concentration at the treatment site, systemic administration of medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment, because smaller total levels of medication are administered in comparison to systemic dosages, and this medication is concentrated at a specific site. Local delivery thus produces fewer side effects and achieves better results. Briefly, a solution that includes a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent. The solvent is then allowed to evaporate, leaving on the stent surface a coating of the polymer and the therapeutic substance impregnated in the polymer. Once the stent has been implanted at the treatment site, the therapeutic substance is released from the polymer coating over time.
Although using coated stents improves pharmacological treatment of the patient and improves biocompatibility of the medical device in the patient, coatings can still be improved. In particular, it is desirable to have biologically beneficial (biobeneficial) stent coatings that are creep compliant and are capable of providing modulated drug release rate by increased water absorption in the overall coating. The embodiments of the present invention provide stent coatings that have these and other advantages.